What brake pressure source remains functional after the loss of hydraulic systems A and B?

The brake accumulator remains functional after the loss of hydraulic systems A and B because it is designed to provide a backup source of hydraulic pressure specifically for critical systems like braking. The accumulator stores hydraulic fluid under pressure, which can be utilized when the primary hydraulic systems are compromised. This feature is essential for ensuring that the aircraft can still safely decelerate and stop in the event of hydraulic system failure.

Other components listed may not maintain their functionality in the event of a hydraulic system failure. For example, the main hydraulic pumps rely on hydraulic system functionality to operate, while the emergency brake system typically uses a different mechanism that may not be available if both primary systems are lost. The battery power supply pertains to electrical systems and would not serve to provide hydraulic pressure for braking. Thus, the brake accumulator is specifically engineered to function in scenarios where other hydraulic sources are unavailable, making it the correct answer.